Abstract-Aqueous activity on meteorite parent bodies is indicated by the presence of carbonates. High spatial resolution ion microprobe analyses of nine individual carbonate grains (four dolomites, five breunnerites) from the Orgueil meteorite reveal linear correlations between 53 Cr excesses and Mn/Cr ratios in all grains, indicative of in situ decay of radioactive 53 Mn (half-life 3.7 million years). The well-defined isochrons appear to have chronological significance. If this is the case, then 53 Mn/ 55 Mn ratios between 2.1 and 4.7 × 10 -6 are inferred for the time of carbonate formation and absolute ages of between 4561 and 4565 Ma are calculated (systematic uncertainty of ±2 Ma). Dolomites tend to have formed slightly earlier than the breunnerites. Our data imply extensive aqueous activity on the Orgueil parent body over a period of several million years, starting ~3-4 Myr after formation of the solar system, that most likely was the result of impact heating and latent heat from the decay of radioactive 26 Al and 60 Fe.

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